1. Field of the Invention
The invention relates to a method of manufacturing an impregnation type cathode which can attain an electron emission of a high current density.
2. Description of the Prior Art
In recent years, a high current density, long life time electron gun is required to satisfy demands for high brightness and high resolution of the electron tube. The impregnation type cathode has been widely used in order to satisfy those demands for a high current density and long life time.
The impregnation type cathode is obtained by impregnating an electron emission material (emitter material), comprising mainly, for example, barium, calcium and aluminum, into a porous sintered body pellet metal having a high melting point. This impregnation is generally carried out in a vacuum environment and at a high temperature, and the emitter material is impregnated into void spaces of the porous sintered body. However, not all of the emitter material can be impregnated into the void spaces of the sintered body, and the emitter material which is not impregnated into the void spaces of the sintered body remains on the surface of the sintered body. This remaining emitter material prevents formation of a metal barium mono-atomic layer on the surface of the sintered body, and so generates an unfavorable result in that the emitter characteristics of the electron tube is degraded.
In the past, as described, for example, in Japanese laid-open patent publication No. 60-17831 (1985), in order to remove the remaining emitter material on the surface of the sintered body, many processes have been used, such as mechanical grinding (filing, sand blasting, use of a lapping machine and so on), or cleaning using acid alkali, or ultrasonic cleaning.
According to the conventional method described above, there may arise some problems, for example, the surface of the pellet is marred by many flaws, which are caused by the mechanical grinding and remain on the pellet surface. Also the emission characteristics deteriorates because barium mono-atomic layers are destroyed. Moreover, the pellet size is small, it is very difficult to homogeneously grind the whole surface of the pellet using mechanical grinding, and also mass-production becomes difficult since many pellets can not be ground at the same time. In addition, the emitter material, in general, is subject to attack by moisture, because the barium mono-atomic layer existing on the surface of the emitter material reacts with the moisture, so that the emission decreases at the portions comprised of barium oxide or barium hydroxide. Therefore, it is not desirable to use the above wet cleaning method using water-based solutions such as acid or alkali.